Resonance exists in electrical stepper motors and is a function of the motor's mechanical construction. Resonance can cause the motor to stall at low speeds.
Most full step motor controllers jump the motor to a set minimum starting speed that is greater than the resonance region. Motors that will not accelerate past a relatively low RPM may be stalled due to resonance.
The drive to a motor can be tuned to minimize resonance and optimize smoothness by adjusting circuit elements such as small potentiometers accessible to a user of the unit. Such potentiometers may adjust a DC offset of one or more phase currents. Since tuning is affected by operating current, such circuit elements may have to be adjusted during the configuration or installation process.
There are several methods that one can use to determine the level of motor resonance in a system.
Tachometer Method
Use an oscilloscope to gauge the output of a tachometer attached to the motor shaft. The tachometer will output a DC voltage, proportional to speed. This voltage will oscillate around an average voltage when the motor is resonating. The amplitude of this oscillation will be at its maximum when you run the motor at its resonance speed. The goal of this tuning method is to tune the motor for its lowest oscillation amplitude.
Sounding Board Method
One can practice his or her tuning skills with an unloaded motor placed on a sounding board or table. When one commands a velocity that is near the motor's resonance speed, the phenomenon will cause an audible vibration. The goal of this tuning method is to tune the motor for the least amount of vibration.
Stethoscope Method
When one tunes his or her motor under loaded conditions, one can hear the audible vibration caused by the motor's natural frequency by placing the tip of a screw driver against the motor casing and placing the handle of the screw driver close to one's ear (as you would a stethoscope). You will also be able to hear the different magnitudes of vibration caused by the motor's natural frequency. The goal of this tuning method is to tune the motor for the least amount of vibration.
Touch Method
After one has had some experience with tuning, one should be able to locate the motor's resonance speed by placing your fingertips on the motor shaft and adjusting the motor's velocity. Once the resonance speed is located, one can tune the motor for maximum smoothness in the same way.
Typically, in order to tune the drive to its coupled motor, the drive must be commanded so that the coupled motor runs at maximum roughness for a first speed motor resonance. Then the circuit elements of the drive are adjusted for best smoothness. Then, the drive is commanded to achieve a second speed resonance until the motor again runs rough. Then the circuit elements are again adjusted for best smoothness. Then the steps are repeated until no further improvement is noted.
The above techniques for tuning a drive to its motor present a number of problems. For example, in the above-noted Tachometer Method, an oscilloscope is required which may not always be readily available. Also, with respect to the Sounding Board Method, the motor must be tuned prior to installation which is not always possible. Finally, with respect to the Stethoscope Method, this technique is somewhat dependent on operator skill and is not always possible to employ depending on the level of background noise.
Consequently, there is a need for a quick, easy, and relatively inexpensive method and system tune a drive to its coupled motor.